The present invention relates to an optical scanning apparatus, such as those employed in copying machines or laser printers, for scanning laser beams for recording or displaying an image. More particularly, it relates to an optical scanning apparatus having an optical system for focusing laser beams on a scanning surface of a light-sensitive member such as a drum after the laser beams have been deflected by a deflecting means such as a polygonal mirror.
Optical scanning devices are widely used in various copying machines and laser printers in which an electrostatic latent image is formed on a light-sensitive member.
FIG. 1 schematically shows an entire laser printer as an example. The laser printer 11 is electrically connected to a printer controller 12 such as a work station or a computer through a cable 13 so as to receive image data and print an image.
The laser printer 11 may be provided with a drum 15 which is to be rotated at a constant speed. Around the circumference of the light-sensitive drum 15, there are provided a charge corotoron 16 for uniformly charging a surface of the drum 15, a developing device 17 for developing an electrostatic latent image, a transfer corotoron 19 for transferring the developed toner image on a recording paper 18, a discharge corotoron 21 for discharging the drum surface after the transfer, and a cleaning device 22 for removing toner residual on the drum surface. A semiconductor laser controller 24 turns on/off or modulates a semiconductor 25 in response to image data. A laser beam 26 outputted from a semiconductor laser 25 is introduced onto a polygonal mirror 28 through a shaping optical system 27 composed of lenses and the like. After the beam is reflected at the polygonal mirror 28, the beam is focused onto the light-sensitive drum 15 through a focusing optical system 29.
Since the polygonal mirror 28 is rotated at a high speed by a polygonal mirror drive motor 31, the reflected laser beam is deflected to scan the drum surface between the charge corotoron 16 and the developing device 17. As a result, an electrostatic latent image is formed on the surface of the light-sensitive drum 15 in response to the image data and is developed by the developing device 17.
The semiconductor laser controller 24 is controlled by a system controller 33 for controlling the overall laser printer 11. A sheet feeder system of the laser printer is also controlled by the system controller 33. Namely, a stack of recording papers 18 received in a cassette tray 35 is fed one by one by a feed roller 35 and advanced along a paper path indicated by dotted lines. The paper is first subjected to the toner image transfer through a gap between the light-sensitive drum 15 and the transfer corotoron 19 and is delivered through a fixing device 37 composed of a pair of rolls, so that a toner image is fixed by heat and pressure. The recording paper on which the image has been formed is discharged from a pair of discharge rolls 38 to be received in the discharge tray 39.
An f.theta. lens system is widely used in a focusing optical system 29. The f.theta. lens system has two functions, i.e., to converge laser beams 26 into a light spot on the light-sensitive member, i.e., the light-sensitive drum 15 or the like and to move the light spot at a constant velocity on the light-sensitive drum surface. In many cases, the focusing optical system 29 is arranged so that a position of deflection is conjugate with a position of the light spot on the light-sensitive drum in a plane perpendicular to a surface to be scanned by the laser beam 26. Thus, a so-called surface tilt correcting optical system is optically formed for correcting an inclination of reflective surfaces of the polygonal mirror.
A surface tilt correcting optical system has been conventionally known which is composed of cylindrical lenses or toric lenses. However, in the case where the cylindrical lenses are used, it is difficult to obtain a good focusing performance. In case of the toric lenses, it is very difficult to produce them.
For instance, Japanese Patent Application Laid-Open No. SHO 55-127514 shows a surface tilt correcting optical system using a cylindrical mirror. Also, Japanese patent Applications Laid-Open Nos. SHO 59-84218 and 64-42625 and HEI 3-130717 show an optical system in combination between a cylindrical mirror and an f.theta. lens system.
The combined optical system of the cylindrical mirror and the f.theta. lens system is shown in FIG. 2 based upon the above-described Japanese Patent Application Laid-Open No. SHO 59-84218. In FIG. 2, the same reference numerals as those in FIG. 1 are used to designate the like members and components and explanation thereof will accordingly be omitted. In the optical scanning system shown in FIG. 2, the laser beam reflected from the polygonal mirror 28 passes through a concave lens element 41 and a convex lens element 42 and thereafter reflected by a concave cylindrical mirror to reach the light-sensitive drum 15 to thereby scan the drum surface by every line in the axial direction of the circumferential surface of the drum 15.
The arrangement of (a) the concave lens element 41, (b) the convex lens element 42 and (c) the concave cylindrical mirror 43 may be made by (a) a spherical single lens, (b) a lens having a cylindrical surface and a spherical surface and (c) a cylindrical mirror, or otherwise by (a) a lens having a cylindrical surface and a planar surface, (b) a lens having a first cylindrical surface and a second cylindrical surface having a refractive power in a direction perpendicular to the first cylindrical surface and (c) a cylindrical mirror, in this order. Howewer, the optical system shown in FIG. 2 suffers from, i.e., it is difficult to produce the optical elements and the production is costly since the shape of the convex lens element 42 is complicated.
FIG. 3 is a cross-sectional view showing the optical scanning system shown in the above-described Japanese Patent Application Laid-Open No. SHO 64-42625 in a direction perpendicular to the main scanning direction. The optical system is composed, in order from the polygonal mirror 28, of a first lens group 51 having a negative focal length as a whole, a second lens group 52 having a positive focal length as a whole and a concave reflector element 53 having a generating line in the main scanning direction of the focusing surface with its cross section in a direction perpendicular to the main scanning direction having a quadratic contour. The laser beam reflected at the concave reflector element 53 is used to scan the scan surface 54 of the light sensitive member or the like.
Thus, in this optical system, since the f.theta. lens system is composed of spherical components, it is easy to manufacture the components. However, the second lens group is large in size, which leads to a problem that optical material loss is unduly large in the case where the second lens group is made from a spherical lens.
FIG. 4 shows a primary part of an optical scanning system disclosed in the above-described Japanese Patent Application Laid-Open No. HEI 3-130717. In this system, the laser beam emitted through a first cylindrical lens 61 is reflected at the polygonal mirror, and passes through a second cylindrical lens 63, a cylindrical mirror 64, a third cylindrical lens 65 and a fourth cylindrical lens 66 in order, thereby scanning the scan surface 67.
In the optical scanning system, a large number of cylindrical lenses are used and it is therefore possible to facilitate the manufacture of the system and to overcome the problem of loss of optical material. However, since the focusing optical system is composed of four components, the number of the optical elements and the cost thereof are increased.
In order to cope with such a problem, it has been proposed to use plastic lenses instead of conventional glass lenses. However, if the glass lenses of such an optical system would be simply replaced by plastic lenses, the refractive indexes of the lenses would be considerably changed due to change in temperature and humidity. Accordingly, it would be impossible to manufacture an optical scanning apparatus which ensures high image quality.